The mitogen-activated protein kinase (MAPK) signaling pathway (EGFR>Ras>Raf>Mek>Erk) is a signaling pathway involved in cell proliferation, which has been considered to be important in tumorigenesis in many studies since it was discovered in 1990.
There are three RAF kinase enzymes in humans: A-RAF, B-RAF, and C-RAF (Marais and Marshall Cancer Surv. 27: 101-125, 1996). Since mutations of B-RAF were found at a high frequency in human cancer, RAF proteins were recognized as a critical initiator and promoter of malignancy (Davies, H. et al. Nature 417: 949-954 (2002)).
B-RAF mutations have been identified in approximately 7% of cancers, including 50% to 70% of melanomas, 35% of ovarian cancers, 50% of thyroid cancers, and 10% of colorectal cancers (Tuveson, et al., Cancer Cell. 4:95-98 (2003); and Xing, Endocrine-Related Cancer: 12:245-262 (2005)). Approximately 90% of the mutations occur as a point mutation (V600E) in which a valine residue is changed to a glutamate residue in the kinase domain. This V600E is an important target for the development of cancer therapeutics. A V600E mutation confers an approximately 500-fold increase in kinase activity, resulting in hyperactivation of MEK and ERK and abnormal growth of tumor cells. Until now, about 40 B-RAF mutations have been found (mainly at the activation segment and the glycine-rich G-loop of the kinase catalytic domain). However, the occurrence of mutations other than V600E is fairly infrequent. In corectal cancer, about 10% of B-RAF mutations occur at the G-loop of the kinase domain (Rajagopalan et al., Nature 2002 418, 934).
Recent studies have found that knockdown of mutant B-RAF by siRNA in human melanoma cells inhibits both MEK and ERK, causing growth arrest of tumor cells and ultimately promoting apoptosis (Sharma, et al., Canfer Res. 65:2412-2421 (2005); and Wellbrock et al., Cancer Res. 64:2338-2342 (2004)). In addition, experimental results from short-hairpin RNA xenograft models targeting mutant B-RAF have shown that tumor regression resulting from B-RAF suppression is reversibly regulated (Hoeflich et al., Cancer Res. 66:999-1006 (2006). Taken together, in-vivo B-RAF signaling is strongly associated with tumorigenicity, confirming B-RAF as an important target for cancer therapeutics.